(i) Field of the Invention
This invention relates to the extraction of toxic organic contaminants, e.g., pentachlorophenol, polychlorinated dibenzo-p-dioxins, and polychlorinated dibenzofurans, from treated wood, e.g., utility poles, railway ties, fence posts, etc. This invention also includes such extraction steps and the subsequent photodegradation of such extracted toxic organic contaminants. In addition, this invention relates to the photodegradation of toxic organic contaminants.
Toxic organic contaminants include polychlorinated dibenzo-p-dioxins and polychlorinated dibenzofurans, which are large groups of chloro-organic compounds which have become ubiquitous in industrial societies. Of the various possible isomers of these compounds, the following are reportedly extremely toxic: 2,3,7,8-tetrachlorodibenzo-p-dioxin, 1,2,3,7,8-pentachlorodibenzo-p-dioxin, 2,3,7,8-tetrachlorodibenzofuran, 1,2,3,7,8-pentachlorodibenzofuran, 2,3,4,7,8-pentachlorodibenzofuran, 1,2,3,6,7,8-hexachlorodibenzo-p-dioxin, 1,2,3,7,8,9-hexachlorodibenzo-p-dioxin, 1,2,3,4,7,8-hexachlorodibenzo-p-dioxin, 1,2,3,6,7,8-hexachlorodibenzofuran, 1,2,3,7,8,9-hexachlorodibenzofuran, 1,2,3,4,7,8-hexachlorodibenzofuran, and 2,3,4,6,7,8-hexachlorodibenzofuran.
Polychlorinated dibenzo-p-dioxins and polychlorinated dibenzofurans are known to cause a temporary form of a skin ailment known as "chlor-acne". Also, polychlorinated dibenzo-p-dioxins and polychlorinated dibenzofurans (particularly 2,3,7,8-tetrachlorodibenzo-p-dioxin) have been found to be extremely toxic to certain animals in laboratory studies.
Because of this reported high level of toxicity in a laboratory tests, there is a general concern as to the long-term effects of polychlorinated dibenzo-p-dioxins and polychlorinated dibenzofurans on human physiology. Accordingly, there is an important need to remove or substantially reduce the content of polychlorinated dibenzo-p-dioxins and polychlorinated dibenzofurans from used telephone poles, used railway ties, used fence posts, etc., prior to disposal or reuse of the waste. There is also a need for a process for treating solutions containing toxic organic contaminants, as described above, and including toxic organic contaminants which have been removed from treated wood, so that they can be disposed of safely.
(ii) Description of the Prior Art
Pentachlorophenol-treated utility poles contain high levels of pentachlorophenol and related contaminants, and consequently can not be disposed of in landfill sites. It has been suggested to use bioremediation as a possible way of decontaminating these materials. Poles removed from service have a high pentachlorophenol content, i.e., of the order of about 5,000-27,000 ppm in the outer 20 mm zone. This high level of pentachlorophenol is toxic to most microorganisms which have been suggested for use in the bioremediation process. Accordingly, it is necessary to pre-treat the pole material to reduce the content of such contaminants before biological remediation.
Physical or chemical methods can be used for the pretreatment process. Physical methods, e.g., dilution, i.e., mixing the pentachlorophenol-containing sawdust with large amounts of uncontaminated sawdust or other materials, so that the pentachlorophenol concentration is low enough for the microorganisms to survive, is not feasible economically. It also has the problem of generating a much larger volume of contaminated waste. Therefore, any kind of dilution approach is not considered to be suitable.
Solvent extraction is probably the easiest and most effective laboratory method of removing pentachlorophenol from contaminated wood. However, extraction using organic solvents is also not considered appropriate commercially, because of environmental concerns and the hazards involved in a large scale operation.
Chemical treatment has also been suggested for pre-treating the pentachlorophenol-containing wood before bioremediation. Pentachlorophenol is, however, very stable and only a few systems can modify and/or degrade this molecule. Because of the strong, relatively non-polar covalent C-C1 bonds in pentachlorophenol, removal of the chlorine by hydrolysis is difficult. Pentachlorophenol is an electron-deficient molecule and should be more reactive towards reduction than oxidation. Potassium-graphite-intercalate has been suggested as an agent for dechlorination of a number of compounds including pentachlorophenol and octachlorodibenzo-p-dioxin. This system, however, requires inert atmosphere, high temperature and absolute anhydrous conditions and is impractical for large scale applications.
Electrochemical reduction has been suggested for use for treating waste waters containing low concentrations of chlorinated organics. Such process was considered not suitable since, for electrochemical processes to work, the electrodes must maintain clean surfaces. Moreover, the oil and other contaminants in pentachlorophenol-treated wood would contaminate the electrodes very quickly.
Reductive dechlorination of chlorinated organic compounds by photochemical reactions has also been suggested to detoxify pentachlorophenol-containing materials. Photochemical degradation of pentachlorophenol and lower chlorophenols in the presence, or absence, of various photosensitizers and catalysts have furthermore been suggested. It is known that polychlorinated biphenyls may be dechlorinated in the presence of visible dyes and amines using visible light.
Oxidation of chlorophenols by enzymes has also been suggested. Laccases may be used to remove chlorophenols PG,6 from water through polymerization. This method, however, does not provide a permanent solution to the problem. The oxidation of phenolic pollutants by lignin peroxidase, an enzyme from Phanerochaete chrysosporium, has also been suggested. On the other hand, it is known that chlorophenols could be converted to much more toxic polychlorodibenzo-p-dioxins by peroxidase catalyzed oxidation.
Supercritical fluids have also been suggested to extract cellulosic materials. A supercritical fluid (SCF) is a fluid at a temperature above its critical value. An SCF has properties which are intermediate between those of gases and liquids. It has a viscosity which is lower than that of a liquid and a density which is higher than that of a gas. These properties allow SCFs to penetrate matrices easily, while retaining reasonable dissolving power. Supercritical fluid extraction (SFE) is a technique in which gases are compressed under supercritical conditions to form a fluid, which is then used to remove chemicals from a matrix. Among the various solvents suitable for SFE, carbon dioxide is the most commonly used, because it is non toxic, non-flammable, and inexpensive. Carbon dioxide also has low critical temperature and pressure, thus having a minimum requirement for equipment design. SFE provides superior extraction to routine solvent extraction in several aspects. For example, SFE leaves no solvent residue in the matrix after extraction, since carbon dioxide is a gas at normal temperature and pressure. The extract is automatically separated from the solvent when the pressure is released (carbon dioxide under noncritical conditions can hardly dissolve any of the extract), and since it eliminates the solvent-extract separation step, it is very energy efficient. In addition, SFE can be done in a closed system where carbon dioxide is continuously recycled.
Supercritical fluid technology has been applied to materials processing and pollution control. For example, it is known that supercritical ethylene may be used to remove trichlorophenol from soil. It is also known that various supercritical fluids, including carbon dioxide may be used to extract organic materials from tar sands. In addition, it is known that supercritical fluids including carbon dioxide may be used to remove hazardous organic materials from environmental solids, e.g. such as soil.
SCF extraction has been particularly useful for obtaining aromatic and lipid components from plant tissues. For example, the oil industry relies extensively on processes by which vegetable oils, e.g., soybean, cottonseed and corn oils, are removed from their vegetative components. The coffee industry uses supercritical processes for removing caffeine from coffee, and flavor extraction using SCFs has been applied to, e.g., hops, vegetables, fruits (lemons), and spices. SCF extractions have also been used to extract fragrances.
Various other uses of supercritical fluids in the processing of materials are now known. For example, supercritical carbon dioxide has been used to remove tall oil and turpentine from coniferous woods; to extract lignin from the black liquor produced by the Kraft process for pulp production; to treat refinery sludges; to regenerate absorbents used in waste water treatment systems; to sterilize pharmaceuticals; to remove off-flavor materials from textured vegetable products; to remove gamma-linolenic acid from fruit seeds; and to decaffeinate coffee; to treat citrus wastes to obtain essential oils by cooking the citrus wastes in the aqueous phase under autogenous pressure at a temperature of about 350.degree. C. to 750.degree. C., in the absence of air or oxygen; to extract of animal-derived materials for enzymatic treatment, e.g., endogenous and/or exogenous enzymatic treatment; for supercritical extraction of essential oils from plants with carbon dioxide for preparing pharmaceutical products; for the isolation of diosgenin, a building block for sterols from plant cell culture; and for the solubilization of biomolecules, e.g. sterols, in carbon dioxide based supercritical fluids.
Ritter et al, in paper entitled "Supercritical Carbon Dioxide Extraction of Simultaneous Pine and Ponderosa Pine", Wood and Fiber Science , Jan 1991, V.23 P.98 et seq, described the extraction of pine wood and bark using supercritical carbon dioxide. The authors also taught that the addition of ethanol to bark prior to the supercritical carbon dioxide extraction produced higher yield of extracts relative to extraction without the addition of the ethanol.
The patent literature is also replete with teachings of SFE extraction procedures. Fremont, in U.S. Pat. No. 4,308,200, taught a process for the extraction of tall oil and terpentine from coniferous woods with fluid carbon dioxide and other supercritical fluids.
Kamarei, in Canadian Patent No. 1,270,623, patented Jun. 26, 1990, provided a process for the supercritical fluid extraction of animal-derived material.
U.S. Pat. Nos. 4,338,199 and 4,543,190 to Modell, described processes in which organic materials were oxidized in supercritical water. U.S. Pat. No. 4,338,199 included a general statement that its process could be used to remove toxic chemicals from the wastes generated by a variety of industries including forest product wastes and paper and pulp mill wastes. U.S. Pat. No. 4,543,190 described the treatment of various chlorinated organics other than dioxins with supercritical water and stated that conversion of these materials to chlorinated dibenzo-p-dioxins were not observed.
U.S. Pat. No. 5,009,746, patented Apr. 23, 1991 by Hossain et al, provided a method for removing polychlorinated dibenzofurans from secondary fibers by contacting the secondary fibers with supercritical or near supercritical carbon dioxide for a period of time at a temperature, pressure, and carbon dioxide flow rate such that a substantial reduction in the level of polychlorinated dibenzofurans associated with the fibers was achieved, and the properties of the fibers, e.g., their physical and chemical properties, were not substantially degraded. The operating conditions taught included: the use of pressures above about 60 atmospheres; temperatures above about 25.degree. C.; carbon dioxide flow rates in the range from about 0.01 standard liters/minute/gram of dry secondary fiber (slpm/gm) to about 10 slpm/gm; and processing periods of from about 1 minute to about 3 hours.
U.S. Pat. No. 5,009,746, patented Apr. 23, 1991 by Hossain et al, provided a method for removing stickies from secondary fibers by contacting the secondary fibers with supercritical or near supercritical carbon dioxide for a period of time at a temperature, pressure, and carbon dioxide flow rate such that a substantial reduction in the level of stickies associated with the fibers was achieved, and the properties of the fibers, e.g., their physical and chemical properties, were not substantially degraded.
U.S. Pat. No. 5,074,958, patented Dec. 24, 1991 by Blaney et al, provided a method for removing polychlorinated dibenzofurans from secondary fibers by contacting the secondary fibers with supercritical or near supercritical carbon dioxide or propane for a period of time at a temperature, pressure, and carbon dioxide or propane flow rate such that a substantial reduction in the level of polychlorinated dibenzofurans associated with the fibers was achieved, and the properties of the fibers, e.g., their physical and chemical properties, were not substantially degraded. That patent also taught a method for removing stickies from secondary fibers by contacting the secondary fibers with supercritical or near supercritical carbon dioxide or propane for a period of time at a temperature, pressure and carbon dioxide or propane flow rate such that a substantial reduction in the level of stickies associated with the fibers was achieved, and the properties of the fibers, e.g., their physical and chemical properties, were not substantially degraded.
U.S. Pat. No. 5,213,660, patented May 25, 1993 by Hossain et al, provided a method for removing polychlorinated dibenzofurans from secondary fibers by contacting the secondary fibers with supercritical or near supercritical carbon dioxide for a period of time at a temperature, pressure, and carbon dioxide flow rate such that a substantial reduction in the level of polychlorinated dibenzofurans associated with the fibers was achieved, and the properties of the fibers, e.g., their physical and chemical properties, were not substantially degraded.
It is now known that the solubility of various chemicals in supercritical carbon dioxide is directly related to the temperature and pressure being used, as well as to the presence of different co-solvents, called "entrainers". It is known that the extraction efficiency and selectivity can be optimized by adjusting these parameters, i.e., temperature, pressure and entrainers.
Kumar et al, in a paper entitled "Effect of Fatty Acid Removal in Treatability of Douglas Fir", presented to The International Research Group on Wood Preservation, Section 4, "Process", Document No. IRG/WP 93-40008, reported on the extraction of fatty acids using supercritical carbon dioxide. The extraction was carried out using supercritical carbon dioxide and methanol or methanol and formic acid as co-solvents. The authors suggested that the addition of co-solvents in supercritical carbon dioxide extraction increases the solventing properties of the supercritical fluid.
Following up on these general teachings, U.S. Pat. No. 5,252,729, patented Oct. 12, 1993 by De Crosta et al, provided two extraction processes. One process was for extracting a compound from plant material by contacting hydrolyzed plant material with a supercritical fluid, optionally with a co-solvent, and recovering the compound from the supercritical fluid. A second process was for removing a compound from plant material, by contacting the plant material with an acid, a supercritical fluid and a co-solvent, and recovering the compound from the supercritical fluid.
That patentee also taught that the hydrolyzed plant material can be prepared by treatment of fresh or dried plant material with acid under conditions effective to promote hydrolysis. Useful acids for hydrolyzing the plant material taught by such patentee included mineral acids, e.g., sulfuric acid, hydrochloric acid, or phosphoric acid, or organic acids, e.g., formic acid, acetic acid, propanoic acid, butyric acid, o-, m- or p-toluene sulfonic acid, benzoic acid, trichloroacetic acid, trifluoroacetic acid; or mixtures of any of the above acids.
That patentee also taught that, optionally, a base could be added during or at the completion of hydrolysis of the root to neutralize any excess acid. Suitable bases, as taught by that patentee, included hydroxides, carbonates and bicarbonates of an alkali metal, e.g., sodium, lithium, or potassium, or of an alkaline earth metal, e.g., calcium or magnesium.
That patentee further taught that representative extracting (solvating) mobile phase components includes the elemental gases, e.g., helium, argon, nitrogen, and the like; inorganic compounds, e.g., ammonia, carbon dioxide, water, and the like; organic compounds, e.g., C.sub.1 to C.sub.5 alkanes or alkyl halides, e.g., monofluoro methane, butane, propane carbon tetrachloride, and the like; or combinations of any of the above.
The patentee also taught that the supercritical fluid could be modified by the addition of inorganic and/or organic modifiers, e.g., compounds as listed above. The patentee taught that the most preferable supercritical fluid was carbon dioxide admixed with chloroform.
That patentee further taught the use of a co-solvent which should be compatible with the supercritical fluid selected and should also be capable of at least partially dissolving the compound being extracted. Suitable co-solvents for use in conjunction with the supercritical fluid as taught by that patentee included aromatics, e.g., xylene, toluene and benzene; aliphatics, e.g., C.sub.5 to C.sub.20 alkanes including hexane, heptane and octane; water; C.sub.1 to C.sub.10 alcohols, e.g., methanol, ethanol, propanol, butanol and isopropanol; ethers; acetone; chlorinated hydrocarbons, e.g., chloroform, carbon tetrachloride or methylene chloride; or mixtures of any of the above. The co-solvent was said to be employed in amounts effective to aid in the wetting and/or hydrolysis of the plant material, and can range from excess to about one volume of solvent per one volume of acid, preferably from about 10 to one volume of solvent per one volume of acid.
The operating conditions taught by that patentee included the contacting with the supercritical fluid at temperatures ranging from about 30.degree. C. to about 300.degree. C., preferably from about 75.degree. C. to about 250.degree. C. The pressure employed was said to be sufficient to maintain the supercritical fluid, and was said to be able to be increased from ambient atmospheric pressure to about 400 atmospheres or more, preferably between about 100 and 300 atmospheres.
Accordingly, it would appear that fluid extraction using supercritical fluid (SFE) should be a viable procedure for reducing the toxic chemicals present in the wood, e.g., waste wooden pole materials and used railway ties. It has been found, however, that the extraction of toxic chemicals from wood, e.g., utility poles and used railway ties is not very efficient.
It is thought that the degradation of pentachlorophenol, polychlorinated dibenzo-p-dioxins and polychlorinated dibenzofurans in solution and in sawdust slurry may be achieved by photochemical reactions. However, a commercially-viable such photochemical degradation has not been taught by the prior art.